Ethereal

Ethereals employ Mind Control just like Sectoid Commanders, but are far more durable. They can also occasionally reflect energy weapon shots back at their attackers. Ethereals are usually guarded by Muton Elite squads. They are the deadliest of opponents.

Before initiating contact, try to have your entire squad nearby ready to flank (flanking reduces the chance of the Ethereal reflecting your shots). For guaranteed damage, use a grenade or rocket (especially good if bunched up with other aliens). If you accidently trigger the Ethereal group and are not ready, run out of it's line of sight and regroup. Set up an ambush for the Elites that usually accompanies the Ethereal and pick them off. When only the Ethereal is left, overwhelmit with numbers.

In preparation for the 'Xavier' (Mind control an Ethereal) achievement, choose a designated soldier with Psionic abilities who has Will above 90. Give the solider Psi Armor and Mind Shield. In game, try to get the Ethereal below 4 health bars and ensure your designated soldier has the 'Psi Inspiration' boost before mind controlling.